1. Field of the Invention
The present invention relates to a varifocal lens barrel applied to various still cameras and movie cameras.
2. Description of the Related Art
A varifocal lens barrel is a kind of lens barrel in which the focal plane (of the varifocal lens system incorporated therein) moves in the optical axis direction when zooming (varying of the focal length) is carried out. The varifocal lens barrel is provided with a focus compensation mechanism for compensating the moving amount of the focal plane to carry out focusing (on an object to be photographed) during zooming. The varifocal lens barrel moves a plurality of lens groups in the optical direction during zooming, and for that purpose, all (or some) of the lenses are in cam engagement with a zoom cam ring so as to integrally rotate with a zoom operational ring. Thus, the moving position of each lens group relative to a fixed barrel of the lens barrel is controlled by rotation of the zoom cam ring. A lens group(s) (of the lens system provided in the lens barrel) in which, due to design issues with the lens barrel therefor, it is difficult to engage with a zoom cam ring of the lens barrel via cam engagement utilize another separate cam ring that integrally rotates with the zoom cam ring, so that that such a lens group(s) can be moved along the optical axis direction via cam engagement with this separate cam ring. Japanese Patent No. 3,490,957 discloses a configuration in which three lens groups (first through third lens groups) of four lens groups (first through fourth lens groups) of a varifocal lens system are in cam engagement with a zoom cam ring, and one lens group (fourth lens group) is in cam engagement with another separate cam ring.
The moving amount of the focal plane of a varifocal lens system varies in accordance with the variation of the focal length during zooming, and therefore, focusing cannot be carried out by a single rotation of the focus operational ring by a predetermined angle. Thus, the focus compensation mechanism requires a special function for varying the moving amount of the focusing lens group in the optical axis direction, in accordance with the variation of the focal length. In order to accomplish this function, the above-mentioned Japanese Patent No. 3,490,957 discloses a focus cam barrel that is operatively associated with the rotation of a zoom cam barrel, and focusing lens group is in cam engagement with this focus cam barrel. When the zoom cam barrel rotates, the position of the operatively associated focus cam barrel changes in the optical axis direction, and during focusing, the focusing lens group rotates about the optical axis while moving along the optical axis direction, relative to the focus cam barrel. Provided the cam shape of the focus cam barrel is defined appropriately, the focusing lens group can move by a different moving amount in the optical axis direction during zooming, whereby an accurate focusing operation can be accomplished through single rotation operation of the focus operational ring by a predetermined angle. Note that although the term “zoom (zooming)”, in the strict sense, normally refers to “zooming by a zoom lens system” in which the focal position does not vary regardless of a variation of magnification, the description of the present invention uses the term “zoom (zooming)” as “variation of magnification by a varifocal lens system.”
3. Problems to be Solved by the Present Invention
According to the lens barrel of the above-mentioned Japanese Patent No. 3,490,957, during zooming, the moving amount of each lens group in the optical axis direction is controlled relative to the fixed barrel of the lens barrel. However, when the focal length ratio of the maximum focal length to the minimum focal length (also referred to as “zoom magnification” or “zoom ratio”) increases, the moving amount of each lens group in the optical axis direction during zooming increases. To cope with this problem, the length of the zoom cam barrel in the optical axis direction needs to be increased, however, this causes difficulties in providing a high zoom-magnification zoom lens barrel with shorter length in the optical axis direction (i.e., hinders miniaturization of the high zoom-magnification zoom lens barrel). A multi-stage cam ring can used as an alternative structure in which one lens group is moved relative to another lens group in the optical axis direction to serve as a zooming mechanism. However, if the number of stages of the cam ring increase, there is an increased risk of assembly error in the plurality of cam rings, and the cam engagement between each cam ring also causes an accumulation of error in the plurality of cam rings. This would worsen the assembly error and deteriorate the overall operation of the lens barrel, which would further increase the risk of unstable imaging performance, inappropriate zooming and inappropriate focusing.
According to the lens barrel of the above-mentioned Japanese Patent No. 3,490,957, the focusing lens group is in cam engagement with the focus cam ring, and during focusing, the positioning of the focusing lens group in the optical axis direction is carried out by rotating the focusing lens group about the optical axis relative to the focus cam ring (known as a rotatable telescoping lens mechanism). This rotatable telescoping lens mechanism causes decentering of lens optical axis due to rotation of focusing lens group (inconsistent alignment of lenses or lens frames), resulting in unstable imaging performance during photography.